Temperature compensation of permeability tuned circuits



Sept. 6, 1955 M HODGlN ET AL 2,717,364

TEMPERATURE COMPENSATION OF PERMEABILITY TUNED CIRCUITS Filed Feb. 5, 1951 ifii 2 INVENTORS.

DAV/D M. H00 1 BY LEO V MIFFLIN A T TORNE Y United States PatentO TEMPERATURE COMPENSATION OF PERME- ABILITY TUNED CIRCUITS David M. Hodgin and LeoV. Mitllin, Cedar Rapids, Iowa, assiguors to Collins Radio. Company, Cedar Rapids, Iowa, a corporation of Iowa Application February 5, 1951, SerialNo. 209,498

5 ClaimS.. (Cl. 33630) This invention relates to a slug tuned inductance which has a substantially constant temperature versus frequency deviation characteristic whether the core is in or out.

Where a permeable core is used to tune an inductance coil the frequency variation with temperature is not equal for the core inserted and the core removed positions. This results in difiiculty in calibrating the inductance of the coil. It is desirable to have the frequency deviation versus temperature curve independent of the core position.

It is an object of this invention, therefore, to provide a temperature compensated core to be used in varying inductance such that variation of temperature will be the same at all positions of the core.

Yet another object of this invention is to provide an improved slug-tuned inductance which is compensated for temperature variations.

A feature of this invention is found in the provision for a slug-tuned coil which has a slug formed into two portions with an adapter fastening the two portions together and compensated for temperature variations.

Further objects, features, and advantages of this invention will become apparent from the following description and claims when read in view of the drawings, in which:

Figure 1 is a perspective view of the variable inductor of this invention; and,

Figure 2 is a sectional view taken on line 22 of Figure 1.

Referring to Figure 1, a base plate has an upwardly extending portion 11 at one end thereof to which is mounted a coil form 12. A coil of wire 13 is wound about the form 12 to form the inductance element of the tuner. As shown in Figure 2, the form 12 is hollow. A lead screw 14 is threadedly received through the portion 11 and has a knob 16 fastened to its outer end. The opposite end is fastened to a core, designated generally as 17. Rotation of the knob 16 moves the core 17 along the longitudinal axis of core 12, thereby varying the inductance of coil 13. The core 17 might be made of pressed-powdered iron. Thus, means are provided for varying the inductance of coil 13 by turning the knob 16 to change the position of the core 17 relative to the coil 13.

The variation of inductance with temperature of coil 13 is not the same for the core-out and core-in positions if the core 17 is a solid piece of powdered iron.

The reason for this is that the permeability of the core varies with temperature and with the core inserted, the effective inductance of the coil is changed. If the core is out of the coil, then its permeability change does not effect the inductance of the coil.

The core 17 of applicants invention comprises two end segments or portions 18 and 19 which are formed with cylindrical openings 21 and 22 at adjoining ends. Cylindrical adapters 23 and 24 are forced into the openings 21 and 22 at adjoining ends. Cylindrical adapters 23 and 24 are forced into the openings 21 and 22 and are formed with longitudinally threaded openings for receivice ing therein the opposite threaded ends 26 and 27 of a coupler 28. The longitudinal position ofv the portion 18 may thereby be adjusted relative to themember 19. A gap is formed between the'members 18 and 19 about the outer periphery and by rotating the members 18 and 19 relative to each other the length of this gap may be ad justed. The coupler 28 and adapters23 and 24, may be made of metal, as for example, copper.

Normally with the core inserted into the coil a temperature change causes a greater inductance variation than occurs when the core is removed. With the gap formed between the members 17 and, 18 a temperature rise causes the coupler 28 to expand, thus increasing the. size of the gap. The increase in the air gap reduces, the effective permeability of the core, thereby compensating for the increase in permeability which is due to the natural temperature coefficient of the permeability of the material. It is to be understood that there is a differential expansion that effects the members 18 and 19 in that the core material expands so as to close the gap whereas the coupler expands so as to widen the gap. The material of the coupler must be chosen so as to give the desired expansion. It is characteristic of some cores to give an increase in permeability when a temperature increase occurs. On the other hand, some materials have a decreaes in permeability when a temperature increase occurs. The material of the coupler 28 may be chosen so as to compensate for either effect.

It is seen that this invention provides means for compensating for permeability variation with temperature in a core.

Although the invention has been described with respect to a preferred embodiment thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claims.

We claim:

1. A movable tuning core structure having a substantially constant temperature coefficient of permeability over its range of movement comprising a cylindrical core formed into first and second axially spaced segments of relatively high permeability, said segments formed with axial recesses, and a coupler of low permeability between said segments having a temperature coefiicient of expansion different from that of said segments and having opposite ends received into said recesses to adjust the longitudinal positions of said segments so that they are physically separated an amount which varies with temperature.

2. A movable core inductor with a substantially constant temperature coefiicient of inductance throughout the range of movement of the core comprising, a support means, an inductance coil and coil form supported on said support means and extending therefrom, a shaft extending through said support means coaxially of said coil, a core mounted on and movable with said shaft axially of and within said coil, said core comprising two spaced cylindrical portions, a coupler joining said two portions but maintaining an air gap therebetween, said portions and said coupler having different temperature coefficients of expansion whereby said air gap varies with temperature.

3. Means for varying the permeability versus temperature characteristic of a tuning core comprising, a core formed with first and second cylindrical portions of high permeability which are formed with axial openings extending partially therethrough, first and second cylindrical adapters forced into the openings formed in said first and second portions, said adapters formed with threaded axial openings, and a cylindrical shaped coupler having a large temperature coefficient of expansion relative to that of said portions, said coupler being of low permeability and formed with a threaded portion at either end thereof and threadedly received in said adapters to hold the first and second cylindrical portions physically apart.

4. Means for eliminating the effect of permeability changes with temperature over a wide tuning range for an inductively tuned oscillator circuit comprising, a movable core in a coil and consisting of a first portion of high permeability material, a second portion of high permeability material, a member of low permeability material between said first and second portions to fasten them together and vary the distance between them in proportion to temperature.

5. A movable core in a variable inductor comprising, a first portion of high permeability with a cylindrical shape and formed with a recess in one end, a second portion of high permeability with a cylindrical shape and formed with a recess in one end, a low permeability coupler fixed between said first and second portions in their recesses to maintain a low permeability gap between them, and said coupler constructed of a material with a References Cited in the file of this patent UNITED STATES PATENTS 548,230 Shallenberger Oct. 22, 1895 1,425,858 Hunt Aug. 15, 1922 1,692,474 Traver Nov. 20, 1928 1,913,978 Ewen June 13, 1933 2,234,002 Harvey Mar. 4, 1941 2,371,236 Gille Mar. 13, 1945 2,394,391 Martowicz Feb. 5, 1946 2,437,345 Bell Mar. 9, 1948 2,450,192 Freeman Sept. 28, 194 

